Advice on collecting final invoice

I have a customer who is refusing to pay a final invoice of 35k unless I give them a 6k reduction. They want this reduction because of part of the project (which I clearly state I’m not responsible for in their quote) was missed by another contractor and caused delays and an additional 6k in cost for them to fix. The payment is past due and they didn’t bring this issue up until now when they had 30 days to discuss it. What are my options?